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Radio Fadeouts and Solar Flares

Solar flares produce copious amounts of electromagnetic
radiation, the X-ray component of which increases the ionisation of the
ionospheric D layer. HF communication generally depends on the reflection
of signals from the higher F layer and such signals must travel through
the D layer at least twice. Increased ionisation
results in greater absorption of the signal in
the D layer.

This effect is known as a short-wave fadeout (SWF) or sudden ionospheric disturbance (SID) and is observed
as an increased attenuation of HF signals
particularly at the lower frequencies. The fadeout
follows closely the pattern of the solar flare, being observed
at the same time as the flare. Fadeouts
mostly have a rapid onset of a few minutes and a slower
recovery lasting perhaps an hour (this is highly variable).

A property of SWFs is that they affect the lower HF frequencies
more than the higher ones which may not be affected at all.
The high frequencies are the last to be affected and the first
to recover.

An important feature of SWFs is that the HF circuit is
affected only if there is an ionospheric reflection
point for the signal
in the sunlit hemisphere. No effect is observed if all the reflection
points are located in the night hemisphere which is shadowed from the
X-rays from the flare.

The intensity of flares at X-ray wavelengths
allows us to estimate the extent (both geographical and in frequency) of
a fadeout. This intensity is now measured by satellite and
SWS has established a page on the Internet
to show the extent and location of fadeouts in
near real-time.